This invention relates to the art of radio broadcasting and, more particularly, to providing adaptive correction of carrier signal distortions caused by modulating an RF carrier signal.
It is known that amplitude modulating an RF carrier signal with an amplitude varying signal will result in the output carrier signal being shifted in phase and amplitude from that of the input carrier signal by an amount that varies with the magnitude of the amplitude varying signal. Moreover, if a modulator filter is employed additional distortions, sometimes referred to as linear distortions, will take place. These problems become particularly significant when such transmitters are employed for passing modulated carrier signals such as that employed in digital radio carrier signals such as that employed in digital radio signals. These signals may be considered wideband digital signals and are sometimes known as digital in-band on channel (IBOC) digital radio signals, Digital Radio Mondiale (DRM), etc.
Reference is now made to FIG. 1 which illustrates a prior art pulse duration modulation amplitude modulated transmitter. In this example, an RF signal from an RF carrier signal source 50 is supplied by way of a filter 6 to the input of a power amplifier 33. An audio signal, from an audio source 52 is supplied by way of a filter 27 to a pulse duration modulator circuit PDM. This circuit includes an integrator 29 that provides a sawtooth ramp signal. The integrator provides an output that is supplied to the negative or inverse input of an operational amplifier 28 whereas the positive input thereof receives an output from filter 27. The PDM modulator 30 receives power from a B+ power supply source 31. The modulator output signal is applied by way of a modulator filter 32 to the power amplifier 33. The output of the power amplifier 33 is supplied by way of a transformer 34 through a filter 35 for application to a suitable broadcasting antenna 37.
The prior art PDM AM transmitter disclosed in FIG. 1 is illustrated as an analog circuit. These transmitters are now being employed to transmit digital radio signals, such as those referred to as IBOC signals. Such transmitters may be operated at various power levels, such as at a turn on level at dawn, a higher operating level during daytime, a lower level at dusk (which may be at the same level as that at dawn) and an even lower level during nighttime operation. Such changes in operating power levels result in distortions introduced by the power amplifier 33 as well as the modulator filter 32. This distortion will vary depending on the power levels as well as other causes such as antenna matching, ground conductivity, etc. The distortions caused by the power amplifier 33 result in both phase and amplitude distortions of the modulated carrier signal and these are considered non-linear distortions. The distortions caused by the modulator and the modulator filter may introduce amplitude distortions which are considered as linear distortions.
The present invention is directed toward improvements for compensating or correcting for the distortions introduced in the output carrier signal caused by the amplitude modulation thereof.